1. Field of the Invention
The present invention relates to a laser system that has an external cavity type semiconductor laser, in particular, a wavelength determining apparatus, a wavelength determining method, a semiconductor laser controlling apparatus, and a semiconductor laser controlling method that allow an oscillation mode of an external cavity type semiconductor laser to be stable.
2. Description of the Related Art
In recent years, since laser systems have features of small size, low power consumption, and so forth, they have been widely used for information devices. For example, a single mode laser has been used for a homographic data storage (HDS). In the HDS, one laser light beam is sprit into two laser light beams by a beam splitter. Thereafter, the split laser light beams interfere with each other on a recording medium so that data are recorded.
As light sources for which holograms are recorded and reproduced, a gas laser and a second harmonic generation (SHG) laser that are single mode light sources are often used. When a semiconductor laser such as a laser diode (LD) that oscillates in a multi-mode is combined with an external cavity, the semiconductor laser can be used in a single mode. As a result, the semiconductor laser can be used for a light source with which holograms are recorded and reproduced.
Next, with reference to FIG. 1, the structure of a typical Littrow type laser system including an external cavity type semiconductor laser of the related art will be described. FIG. 1 is a plan view showing a laser system 200. The structure of the laser system 200 is the same as the structure of a laser system described in Non-Patent Document 1.
[Non-Patent Document]
L. Ricci, et al.: “A compact grating-stabilized diode laser system for atomic physics,” Optics Communication, 117 1995, pp 541-549.
In the laser system 200, a laser light beam of multi-longitudinal mode (oscillation light beam) emitted from a semiconductor laser device such as a laser diode 201 is collimated by a lens 202 and then entered into a grating 203. The grating 203 outputs a first-order diffracted light beam. A first-order diffracted light beam having a predetermined wavelength that depends on the alignment angle of the grating 203 is reversely injected to the laser diode 201. As a result, the laser diode 201 resonates with the injected first-order diffracted light beam and emits a single mode light beam (zero-th order light beam denoted by arrow F in FIG. 1). The wavelength of the zero-th order light beam is the same as the wavelength of the light beam returned from the grating 203.
In the example, with a combination of a screw 205 and a piezoelectric device, the angle of the grating 203 is precisely adjusted.
Next, with reference to a graph shown in FIG. 2, the relationship between the laser power of a laser light beam that is output from the external cavity type laser system described in FIG. 1 and the wavelength of the laser light beam will be described. The horizontal axis of the graph shown in FIG. 2 represents the laser power in mW, whereas the vertical axis of the graph represents the wavelength in nm. FIG. 2 shows that as the laser power of laser light beam increases, the wavelength of the laser light beam vary nearly in a saw shape.
The external cavity type laser system has a mode hop region of an external cavity and a mode hop region of a semiconductor laser chip. In the mode hop region of the external cavity, as the laser power increases, the wavelength of the laser light beam that is emitted gradually increases. In the mode hop region of the semiconductor laser chip, as the laser power increases, the wavelength of the laser light beam that is emitted sharply decreases. As the laser power increases, the wavelength of the laser light beam discretely varies to some extent.
When the laser power is around 30 mW, a laser light beam having a single wavelength is emitted as a perfect single mode. However, when the laser power is around 32 mW, a laser light beam that has three modes are generated. When the laser power is around 35 mW, namely in the mode hop region of the semiconductor laser, the laser light beam has three modes around wavelength of 409.75 nm and three modes around wavelength of 409.715 nm, a total of six modes.
FIG. 3A, FIG. 3B, FIG. 3C, and FIG. 3D show spectrums of several laser light beams. As described above, in the mode hop region of the external cavity, the wavelength of the laser light beam gradually increases and the laser light beams have spectrums as shown in FIG. 3A, FIG. 3B, and FIG. 3C. However, in the mode hop region of the laser chip of the semiconductor laser, the laser power is around 35 mW and a laser light beam has a spectrum as shown in FIG. 3D.
When these laser light beams are used for the HDS, a laser light beam having three modes with a laser power of around 32 mW (as shown in FIG. 3A) and a laser light beam having two modes (as shown in FIG. 3B) have the same recording and reproducing characteristics as a laser light beam having a single mode (with a spectrum shown in FIG. 3C). Thus, these laser light beams can be used in the same manner as a laser light beam having a single mode. In this example, the perfect signal mode that takes place in a laser light beam having a laser power of around 30 mW and the three modes and two modes that take place in laser light beams having a laser power of around 32 mW are referred to as the usable mode as a general term.
On the other hand, as shown in FIG. 3D, a laser light beam having six modes that takes place with a laser power of around 35 mW is similar to two sets of three modes, the two sets being apart from each other by around 40 pm. Thus, when data are recorded on a hologram medium with a laser light beam having a usable mode, the M/# of the hologram medium is 6.5. If data are recorded on this hologram medium with a laser light beam having an unusable mode, the M/# of the hologram medium deteriorates and decreases to 2.5.
The M/# is named the M number that is one of important factors used to evaluate the characteristics of the medium. In other words, when hologram data are recorded, a light beam emitted from the same laser light source is divided into two light beams by the beam splitter. Two light beams of a record light beam and a reference light beam are reflected by the mirror and emitted to the same position of the medium. When hologram data are reproduced, only the reference light beam is emitted to the same position of the medium in the same manner as the hologram data are recorded and a diffracted light beam is obtained. With the incident light amount and the diffracted light amount of the reference light emitted to reproduce the hologram data, the diffraction efficiency is defined as follows.Diffraction efficiency=diffracted light amount/incident light amount
The M/# is defined as follows.M/#=Σ(root of diffraction efficiency)
where Σ is the sum of diffraction efficiencies in the case that hologram data are multiplex-recorded at the same position. The root would be necessary from an optical view point. As is clear from the foregoing formula, when the number of multiplexing times is large, the sum becomes large. Thus, the M/# increases. In addition, when hologram data are strongly recorded, the diffraction efficiency becomes large. Thus, the M/# increases. In other words, as the M/# becomes larger, the medium can be more suitably used to multiplex-record hologram data.
The region in which a laser light beam having a usable mode is obtained nearly matches the mode hop region of the external cavity. The region in which a laser light beam having an unusable mode is obtained nearly matches the mode hop region of the laser chip of the semiconductor laser. As is clear from the graph shown in FIG. 2, since the region in which a laser light beam having a usable mode is obtained is much wider than the region in which a laser light beam having an unusable mode is obtained. Thus, if a laser light beam having an unusable mode can be effectively eliminated, the external cavity type semiconductor laser can be used for the HDS.
In addition, the relationship between the laser power and the wavelength of a laser light beams vary with the internal temperature of the external cavity type laser. When the temperature of the external cavity type semiconductor laser is not constant, the position of the laser power with which the laser light beam has an unusable mode varies. Thus, in the related art, the temperature of the external cavity type semiconductor laser is kept almost constant (within a deviation of 10 mK) and the region of a laser light beam having an unusable mode is prevented from varying. In addition, the laser power in the region is not used.